1. Field of the Invention
The present invention resides in the field of superconducting coils and more particularly relates to the reinforcement of superconducting coil magnets using composite superconducting wires that are clad with high-strength materials.
2. History of the Prior Art
Superconductivity is the phenomenon where, when a particular material, such as a wire, is subjected to successively colder temperatures, it undergoes a state transition where all electrical resistance disappears, i.e. the material can conduct electricity without generating any heat. At this stage the material is said to have become a superconductor. This phenomenon only occurs for specific metals/alloys and compounds. The state of superconductivity for a given superconductor is a function of its temperature, background magnetic field, and transport electric current.
Superconducting magnets almost exclusively consist of windings of superconducting wire, also known as coils. In very large magnets the “wire” may take the form of cabled superconducting wires. For some superconducting magnets the cable of wires is contained, together with helium for cooling, in a metal conduit. All wires used in building such magnets include insulation that surrounds the wire to be wound. Turns and layers of the winding of a magnet are often bonded to each other by a bonding agent, such as epoxy. Such windings are said to be impregnated.
In many superconducting magnet applications, for example in nuclear magnetic resonance (NMR) and for magnetic resonance imaging (MRI), the windings are usually solenoids. Often NMR and MRI magnets are comprised of a number of solenoid coils that are electrically connected. The magnetic fields produced by the system of coils combine to produce an intended magnetic field intensity over an intended volume inside the system of coils of the magnet. The field intensity of a solenoid, as well as most other magnet configurations, scales with the number of turns of the wire that makes up the windings of the coil and the electric current (amperes) that passes through the wire. Often it is said that the field intensity scales with “ampere-turns.” For reasons of: 1) minimizing space, 2) using less superconducting wire, and more importantly 3) reducing the stored energy of the coil, most coils used in magnets with high stored energy such as NMR and MRI magnets are designed and built such that a given coil is as compact as possible.
When current passes through the wire of a coil, a magnetic field is produced. When current passes through a system of coils, a combined magnetic field is produced. The magnetic field (B) which is produced by a coil or a system of coils around a given turn inside the winding volume (pack) of a coil interacts with the current in the wire and produces a force on the wire, referred to as the Lorentz Force. Because the orientation of the magnetic field varies according to the location of the turn within the coil, or system of coils, the direction of the force varies also. The Lorentz Forces are balanced by the wire and other materials that make up the coil, resulting in mechanical stresses in the wire. In a given solenoid coil the Lorentz Forces on the turns combine to produce two components of stress in the wire. A tensile hoop stress is developed in the wire of magnitude B×I×R where B is the local field strength, I is the current and R is the radius of curvature. A transverse stress is also incident because radial components of the field in regions of a winding produce an axial force. Of these components of stress the hoop stress is generally the more significant, but both must be considered in the structural design of a high field magnet. When a solenoid coil is charged by current passing through it, there is a balance between its electromagnetic energy and mechanical elastic energy. Therefore a charged coil, or a system of coils, stores energy.
Design parameters and conventional approaches to solenoid design are discussed, for example, in: Superconducting Magnets, M. N. Wilson, Oxford University Press, New York, N.Y. (1983) and Case Studies in Superconducting Magnets, Y. Iwasa, Plenum Press, New York, N.Y. (1994).
The stresses on the wires inside many superconducting coils may reach hundreds of MPa. Therefore a main part of the design optimization of coils is calculation of stresses and bearing of the stresses by the coil.
To achieve the highest possible fields inside the bore of many magnets, for example the NMR and MRI magnets, and most effectively support against Lorentz Forces, the wires used in these magnets are usually a solid monolithic wire, and their winding packs are devoid of any space for helium which is sometimes used as a heat sink inside the winding pack of some superconducting magnets. The coils of many superconducting magnets are comprised of solid matter. These are often referred to as being adiabatic coils as any release of local energy within the winding pack is absorbed by the winding pack itself first, and then it is transferred to the cooling medium that is external to the winding pack.
For reasons that relate to stability of superconductors, superconducting wires used in most magnet application are multifilamentary (MF) composites. [see for example Stability of Superconductor, by Lawrence Dresner, Plenum Press, New York, N.Y., 1995]. The superconducting filaments in MF wires in use in most superconducting magnets today are made from niobium-titanium (Nb—Ti) alloy. The Tc and Bc2 for Nb—Ti are about 10 Kelvin (K) and about 10 (T), respectively. Nb—Ti alloy is ductile and its use in fabricating MF wires, and subsequently in a magnet, is straightforward and comparatively less expensive than using other materials. MF Nb—Ti composite wires are often comprised of Nb—Ti filaments inside a copper or copper alloy matrix.
Superconducting magnets for operation at fields higher than about 10 T rely principally on the use of type A15 superconductors. Among the A15 superconductors the Nb3Sn based wires are most practical for large-scale production, basically due to the fact that fabrication of Nb3Sn is economical and less complicated. Almost all operating A15 magnets to date have used Nb3Sn conductors. The Tc and Bc2 for Nb3Sn are about 18 Kelvin (K) and about 23 (T), respectively. Other A15 superconductors, such as Nb3Al, that have better superconducting properties than Nb3Sn are under development.
Nb3Sn, like other A15 phases, is an intermetallic compound and is inherently brittle. Therefore it does not lend itself to normal conductor fabrication methods where a given material undergoes significant plastic deformation. For most applications in magnet technology, Nb3Sn superconductors are produced by a two-step process in which a multifilamentary composite wire, that contains Nb and Sn in separate regions, is formed into wire (Nb3Sn precursor wire) and then, during a subsequent reaction heat treatment at, for example 650C-750C, the Nb3Sn is formed by solid state reaction. MF Nb3Sn composite wires are often comprised of Nb3Sn filaments inside a copper and bronze alloy matrix. Because Nb3Sn is brittle, windings of Nb3Sn wire are often produced by winding un-reacted wires and then heat-treating the winding as a whole. This approach is referred to as the wind-then-react method. If the winding diameter is large enough the winding can be performed using a reacted Nb3Sn wire. This approach is referred to as react-then-wind method. Because MF Nb3Sn wires have to be heat-treated at temperatures of 650 C-750 C, the non-superconducting matrix in the wires which is most often copper or bronze is in a fully annealed state and is therefore mechanically weak and cannot contribute much to the bearing of Lorentz Forces. Therefore it is particularly desirable to reinforce Nb3Sn wires and coils for superconducting magnet applications.
A main challenge of design and manufacturing coils with high stored energy, for example for NMR and MRI magnets, is to minimize the size of the coils by using conductors that have enough superconducting material to provide the required ampere-turns and optimizing/minimizing structural material to bear the mechanical stresses.
Up to the present time the following methods have been used to reinforce the coils/wires of magnets that use MF wires:    a) extra cold worked (hard) copper has been incorporated into the wire wherein the non-superconducting part of a wire that carries no current is increased and therefore the ampere-turns for a given winding pack is decreased;    b) winding sections have been over-banded with a strong material, including steel, beryllium copper, carbon fiber, etc.; and    c) attempts have been made to incorporate reinforcements in addition to the use of copper and copper alloys inside superconducting wires.
One disadvantage to the use of hard copper as the reinforcement is that the strength of hard copper is limited to about 40,000 psi (275 MPa) in consequence of which in many magnet windings more copper is needed for reinforcement than for stabilization. The result is a lower overall winding current density than could be obtained with the separation of functions, that is, enough copper for stabilization or quench protection and enough high-strength reinforcement for stress containment. A disadvantage of overbanding is that high radial strain in the active winding is needed to transfer the electromagnetic loads to the overbanding. The most efficient and compact winding is obtained when the reinforcement is distributed throughout the active winding. Variations in distributing reinforcement throughout the active winding, though, can create disadvantages inside a superconducting wire in that it complicates the wire manufacturing process for both Nb—Ti and Nb3Sn MF wires because the reinforcing material needs to be both mechanically compatible with the wire manufacturing process which limits the choice of materials as such materials must also be chemically inert to the chemistry of the superconducting wires.